FEMALE chimps keep the noise down down during sex - to make sure their rivals don't get jealous.
Experts from St Andrews University found that the primates lowered their voices to avoid detection while mating.
Researchers studying the sex lives of chimps in the jungles of Uganda also found that female chimps did it with as many males as possible to gain protection from stronger females.
But they always kept the noise down to keep their promiscuity a secret.
The research has also debunked the popular theory that male chimpanzees fight over potential mates.
Instead, females were more concerned about keeping their sex lives a secret from the rest of the group.
Psychologists Simon Townsend and Professor Klaus Zuberbuhler say the study sheds new light on the sophisticated mental capacities and social intelligence of our nearest living relatives.
Simon said: "Competition between females can be dangerously high in wild chimps.
"The female chimps we observed seemed to be much more concerned with having sex with as many different males as possible, without other females finding out about it, than causing male chimps to fight over them.
"We also found that the calling behaviour of copulating females was unrelated to their fertile period and, therefore, not linked to the likelihood of conception."
The researchers worked in collaboration with Tobias Deschner of the Max Planck Institute in Leipzig, Germany.
Prof Zuberbuhler hit the headlines in March when he published findings which suggested that monkeys had developed a primitive language.
In his study of putty-nosed monkeys, he found they produced different alarm calls in response to being disturbed by other species, including leopards and crowned eagles.